/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        //使用快慢指针来解决使用存在环的问题
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast&&slow)
        {
            if(fast->next)
            {
                fast = fast->next->next;
            }
            else {
            break;
            }
            slow = slow->next;
            if(fast == slow)
            {
                return true;
            }
        }
        return false;
    }
};